1. Field of the Invention
The present invention relates generally to a vapor phase growth apparatus arranged to avoid unwanted mixture of reactant and cleaning gases. This invention also relates to a gas mixture avoiding method for use in the vapor phase growth apparatus.
2. Related Art
Prior known on-wafer film fabrication systems typically include a vapor phase growth apparatus using chemical vapor deposition (CVD) techniques, such as vapor phase epitaxial (VPE) growth schemes. An example of such epitaxy apparatus is disclosed in Published Unexamined Japanese patent application No. 9-17734 (“JP-A-9-17734”). The apparatus as taught thereby is designed to form a silicon (Si) film by use of a mixture or “blend” of a gas of reactant species and a carrier gas, such as gaseous silane (SiH4) and hydrogen (H2).
Note here that a polycrystalline silicon or “poly-Si” film is fabricated on a substrate with inferior crystallinity whereas a single-crystalline silicon (Si) film is on single-crystal Si substrate.
An exemplary film fabrication apparatus is shown in FIG. 3. As shown herein, a single-crystal semiconductor wafer 302 is loaded and mounted on a support table structure 301 in a processing chamber 303. This wafer 302 is heated by a heater 304 up to a prespecified temperature high enough to achieve the intended film formation.
The chamber 303 has a gas flow path (first path) as coupled thereto, including a parallel combination of gas feed pipes 305a and 305b. These gas pipes 305a-305b are associated at their far ends with gas cylinders 306a and 306b which contain gases of monosilane (SiH4) and hydrogen (H2), respectively. The SiH4 gas feed pipe 305a is provided with a valve 307a at a midstream location thereof, while the H2 gas pipe 305b has a valve 307b. These valves 307a-307b are each driven to open and close for control of the gas flow rate in pipe 305a, 305b. 
While the valves 307a-307b open, the SiH4 and H2 gases are supplied from the cylinders 306a-306b via the pipes 305a-305b to the interior space of the chamber 303, resulting in a Si film being formed by epitaxial growth on the single-crystal Si wafer 302. Usually this film formation will be repeated a number of times. For example, as suggested by JP-A-2000-282241, when letting the SiH4 gas contain a dopant gas of boron (B), aluminum (Al) or gallium (Ga), it is possible to form a single-crystal semiconductor layer of p-type conductivity; use a dopant gas of phosphorus (P) or arsenic (As) for n-type. Additionally, as taught from JP-A-2004-281673, a silicon dioxide (SiO2) film is formable by supplying SiH4 and O2 gases, and an Si3N4 film is fabricatable by supply of SiN4 and HN3 gases.
The chamber 303 is also equipped with a gas flow path 308 which is used to supply thereto a ClF3 gas—more specifically, a CF4 gas in the case of forming SiO2 films, and C2F6 gas in the case of Si3N4 films. The flow path 308 has a gas flow control valve 309 at its midstream part.
After having formed the intended film (i.e., Si single-crystal film), cleaning is applied to the interior of the chamber 303. During the cleaning, let the valve 309 open whereby a cleaning gas of ClF3 is supplied from a cylinder 310 to the chamber 303 along the gas flow path 308.
In the on-wafer film formation and intra-chamber cleaning processes, the interior of chamber 303 is evacuated by a vacuum pump 313 coupled thereto through a gas exhaust pipe 312 with a valve 311 being opened so that residual gases within chamber 303 are drained away to the outside.
In the vapor phase epitaxy apparatus shown in FIG. 3, a need is felt to prevent unwanted mixture of the SiH4 reactant gas used for film formation and the ClF3 cleaner gas, because such gas mixture poses a high level of danger.
To this end, an attempt is made during cleaning to completely close the valve 306a that is coupled to the gas pipe 305a for flowage of the SiH4 reactant gas used for film formation. What must also be done is to “purge” the chamber interior by a chosen gas, e.g., nitrogen (N2), after completion of the film formation. Thereafter, the exhaust valve 311 is opened to perform vacuuming by the vacuum pump 313 coupled to gas exhaust pipe 312, causing residual SiH4 gas to be discharged from chamber 303 almost completely.
The prior art method has an advantage as to the capability to ascertain the completion of gas exhaust in the evacuation event. Unfortunately this advantage does not come without accompanying penalties, one of which is the lack of an ability to check whether the SiH4 gas leaks or “intrudes” into the chamber during cleaning. In other words, the prior art has no means to detect the intrusion of SiH4 gas into the chamber.
As apparent from the foregoing, prior art vapor phase growth apparatus remains deficient in safety and thus is required for further improvements in system architecture.